In recent years, the telecommunication industry has seen the development and deployment of the first commercial Nongeostationary Orbit (NGSO) satellite constellations to respond to the rapidly expanding demand for global telecommunication services. Such satellite communication systems have the potential to provide worldwide and/or regional coverage at a much lower cost than would be possible using a terrestrial network. Such satellite communication systems also have the potential for providing economical services to virtually any point on the Earth through satellite-to-satellite and satellite-to-ground links, even to remote or sparsely populated areas where it may not be economically feasible to deploy a terrestrial network. Of various satellite communication systems that have been proposed, some propose to provide world-wide coverage, while others propose to provide regional coverage, or coverage within a particular latitude band. Among these satellite communication systems, use of satellite constellations designed with both polar and inclined orbits have been proposed.
In NGSO satellite communication systems, or networks, satellites move at relatively rapid speeds over the surface of an underlying celestial body, such as the Earth. As it sweeps over the surface of the Earth, an NGSO satellite projects a satellite "footprint" made up of a number of radio frequency (RF) "beams" or "cells" toward system users located on or near the surface of the Earth. In this context, a "cell" is defined as the coverage area formed on the Earth's surface by a single antenna beam, and a satellite "footprint" is defined as the aggregation of all cells formed by the antenna/s of a single satellite. A cell contains multiple channels. Desirably, channel access is allocated by frequency, time, by coding, or by a combination of frequency, time, and/or coding to the cells of the system such that communications occurring in adjacent cells use different channels to minimize the effect of interference between channels.
Because each NGSO satellite is visible to a particular system user for relatively small increments of time (typically only for several minutes), communications having a duration of more than several minutes will be handled through a "hand-off" process through the network, which involves switching communication service between cells or beams of a single satellite footprint and between cells or beams of different satellites within the satellite communications system to ensure continuous communication.
Typically, when a hand-off occurs, a communication unit (CU), such as a communication device being used by a system user to communicate through the system is assigned a new time/frequency channel in the next beam or cell. Each such hand-off event requires the exchange of signaling messages between the satellite and the CU.
Also, because hardware resources cannot be simultaneously connected to more than one beam (or cell) it becomes necessary to assign separate hardware resources to each user, or more precisely to each CU employed by each user, as a CU is passed on to different cells of a single satellite and/or to cells of different satellites. Hand-off paths of different users tend to be nonuniform as CUs are handed-off between cells.
Nonuniform hand-off paths among multiple users complicate the hardware resource switching process, complicate satellite payload requirements, and can impose significant burdens on the entire satellite communication system in terms of processing capability and power requirements.
Earth-fixed satellite cells have been proposed to provide a low earth orbit (LEO) satellite communication system which ameliorates the burdens of complex switching arising from frequent hand-offs. An example of an earth-fixed satellite cellular system is described in U.S. Pat. No. 5,408,237 entitled "Earth-Fixed Cell Beam Management for Satellite Communication System." Earth-fixed systems can minimize cell-to-cell hand-off. However, a drawback of earth-fixed satellite cellular systems is that these systems do not compensate for demand variations because the focal direction for projection of a satellite footprint is predetermined. Moreover, each cell in an earth-fixed cellular system has a fixed capacity. Thus, only a number of subscribers not exceeding a particular cell's capacity can use the system at a given time.
Thus, a solution to the problem of providing a high capacity satellite communication system which minimizes the deleterious effects associated with cell-to-cell hand-offs continues to elude the satellite communications industry. Therefore, what is needed is a method and apparatus to simplify the hand-off process in an NGSO satellite communication system.